Short-offset Transient Electromagnetic Transmission System

TECHNICAL FIELD

The disclosure relates to the field of geophysical exploration technologies, and more particularly to a short-offset transient electromagnetic transmission system.

BACKGROUND

Artificial source electromagnetic method (also referred to as controlled-source electromagnetic, abbreviated as CSEM) is one of mainstream methods for mineral resource exploration, which uses grounded wires or closed loops as antennas to excite the ground and extracts information about underground electrical structures by observing the ground response signals. According to the distance between the observation area and the transmission source, it can be generally divided into far-source methods and near-source methods. For a long time, far-source methods (such as controlled source audio-frequency magnetotellurics abbreviated as CSAMT, long offset transient electromagnetic method abbreviated as LOTEM, etc.) have become the mainstream of artificial source electromagnetic methods due to their relatively simple solutions and have been widely promoted worldwide. In near-source observation, although the signal is stronger and the bandwidth is larger, which is more conducive to achieving large-depth and high-precision detection, the wave field conditions in the near-source area are complex, so the development of near-source methods has been relatively lagging. Since 2000, researchers at the University of Edinburgh in the United Kingdom have proposed the near-source method, multi-transient electromagnetic (MTEM) mothed, which is based on multi-channel arrays, achieving fine identification of subsurface geological structures (also referred to as underground structures). Since then, the study of the near-source method has gradually become a research hotspot worldwide. Since 2013, the short-offset transient electromagnetic (SOTEM) method has been proposed, which has the advantages of large detection depth and strong resolution.

Currently, in actual SOTEM observations, although existing electromagnetic equipment can stably obtain data, the actual detection effect cannot meet expectations, indicating that the equipment is not well adapted to the method. From the perspective of equipment, in traditional far-source methods, the transmission and reception distances are large, and the signal amplitude and bandwidth are significantly attenuated. Equipment matching far-source methods mainly considers achieving large-depth detection by increasing transmission power and suppressing sensor noise levels. However, because the signal itself has a limited bandwidth (about 10 kilohertz abbreviated as kHz) under far-source observation conditions, the equipment also only considers a smaller bandwidth in the entire process, that is, existing equipment mainly achieves high-power transmission and low-noise observation under narrow bandwidth conditions. In actual work, existing electromagnetic transmitters have weak adaptability to the SOTEM method, mainly reflected in the bandwidth of the transmission current being smaller than the bandwidth required by the SOTEM method.

In addition, the transmitter of SOTEM typically has a long transmission distance of up to 1 kilometer (km), with wire inductance reaching as high as 2.5-3 millihenries (mH). The characteristic of inductive current lagging behind inductive voltage severely limits the bandwidth of the transmission current. To achieve high-bandwidth current transmission, constant voltage clamping technology is generally used to accelerate the switching speed of the current. Currently, the mainstream electromagnetic equipment on the market usually adopts an active constant voltage clamping scheme, with a clamping voltage of generally 1000 volts (V). To achieve faster switching of the transmission current, it is necessary to increase the clamping voltage. However, the voltage withstand characteristics of power devices limit further increases in voltage, while also posing safety risks.

SUMMARY

Technical problem to be solved by the disclosure is to provide a short-offset transient electromagnetic transmission system.

To achieve the above purpose, the disclosure adopts the following technical solutions.

Specifically, a short-offset transient electromagnetic transmission system, including: an electromagnetic transmitter with four transmission power supplies P 1 , P 2 , P 3 , P 4 , five sets of grounding electrodes A, B, C, D, F, and eight transmission wires LA, LB 1 , LB 2 , LC 1 , LC 2 , LD 1 , LD 2 , LE. The grounding electrode A and the grounding electrode B are connected to the transmission power supply P 1 via the transmission wire LA and the transmission wire LB 1 . The grounding electrode B and the grounding electrode C are connected to the transmission power supply P 2 via the transmission wire LB 2 and the transmission wire LC 1 . The grounding electrode C and the grounding electrode D are connected to the transmission power supply P 3 via the transmission wire LC 2 and the transmission wire LD 1 . The grounding electrode D and the grounding electrode E are connected to the transmission power supply P 4 via the transmission wire LD 2 and the transmission wire LE. The transmission wire LA and the transmission wire LB 1 are pulled from the electromagnetic transmitter to a position of the grounding electrode B in a twisted-pair manner, and the transmission wire LD 2 and the transmission wire LE are pulled from the electromagnetic transmitter to a position of the grounding electrode D in the twisted-pair manner.

In an embodiment, the electromagnetic transmitter includes: a diesel generator, a three-phase rectifier, an inverter isolation power supply, a constant current voltage regulator, and an output inverter. The diesel generator is configured (i.e., structured and arranged) to provide 380 volts (V) three-phase alternating current. The three-phase rectifier is configured to convert the three-phase alternating current into 540 V direct current. The inverter isolation power supply is configured to acquire four sets of 1000 V isolated power supplies through an inverter bridge, four transformers, four high-frequency rectifier bridges, and an inductor-capacitor (LC) filter. The constant current voltage regulator is configured to control an output voltage to achieve constant current regulation. The output inverter is configured to generate the four output transmission power supplies P 1 , P 2 , P 3 , P 4 .

In an embodiment, the output inverter adopts a bootstrap high-voltage clamping technique.

In an embodiment, the four transmission power supplies P 1 , P 2 , P 3 , P 4 are relatively independent, and according to an output load, duty cycle signals are dynamically adjusted by controlling switching devices Q 1 -Q 4 to maintain the output current constant.

The disclosure divides the 1 km long electrode distance into four short electrode distances using five sets of grounding electrodes, and uses eight twisted wires to connect the grounding electrodes to four sets of output power supplies (i.e., four transmission power supplies), designing a bootstrap ultra-high voltage clamping technology to further accelerate the switching speed of the current. By adopting the technical solutions of the disclosure, the electromagnetic transmission system is effectively adapted to the SOTEM method, while achieving rapid switching of transmission current under high power conditions.

BRIEF DESCRIPTION OF DRAWINGS

To more clearly illustrate the embodiments of the disclosure or the technical solutions in the related art, the following will briefly introduce the drawings needed in the description of the embodiments or the related art. Apparently, the drawings in the following description are only some embodiments of the disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

FIG. 1 illustrates a schematic diagram of a relationship between inductance of a 1 km wire and a wire radius.

FIG. 2 illustrates a schematic diagram of a relationship between inductance and length of a wire with a radius of 3 millimeters (mm).

FIG. 3 illustrates a schematic structural diagram of a short-offset transient electromagnetic transmission system.

FIG. 4 illustrates a schematic structural diagram of an electromagnetic transmitter.

FIG. 5 illustrates a schematic structural diagram of an output inverter based on a bootstrap high-voltage clamping technique.

FIG. 6 illustrates a schematic diagram of a charging process of a bootstrap capacitor.

FIG. 7 illustrates a schematic diagram of the bootstrap high-voltage clamping control and output voltage and current waveforms.

FIG. 8 illustrates a schematic diagram of simulation results based on the bootstrap high-voltage clamping technique.

DETAILED DESCRIPTION OF EMBODIMENTS

The following will clearly and completely describe the technical solutions in the embodiments of the disclosure with reference to the accompanying drawings. Apparently, the described embodiments are only a part of the embodiments of the disclosure, not all of them. Based on the embodiments of the disclosure, all other embodiments obtained by those skilled in the art without creative effort shall fall within the protection scope of the disclosure.

To make the above purposes, features, and advantages of the disclosure more obvious and understandable, the following will further describe the disclosure in detail with reference to the accompanying drawings and specific embodiments.

Embodiment 1

The embodiment of the disclosure provides a short-offset transient electromagnetic transmission system, which divides the 1 km long electrode distance into four short electrode distances using five sets of grounding electrodes, and uses eight twisted wires to connect the electrodes to four sets of isolated transmission power supplies, designing a bootstrap ultra-high voltage clamping technology to further accelerate the switching speed of the current.

For a 1 km long wire, if the mutual coupling relationship between the wires and the ground is ignored, the relationship between the wire inductance and the wire radius is as shown in FIG. 1 .

For a copper wire, with a current of 100 Amperes (A), the cross-sectional area is estimated to be 25 square millimeters (mm 2 ), the radius is about 3 mm, and the corresponding inductance is about 2.532 mH. Setting the wire radius to 3 mm, the relationship between the wire inductance and the wire length is as shown in FIG. 2 .

The 1 km electrode distance is evenly divided into four parts by five electrodes, each part is 250 meters (m), and the corresponding inductance of the wire with a radius of 3 mm is 0.5637 mH.

As shown in FIG. 3 , the embodiment of the disclosure provides a short-offset transient electromagnetic transmission system, including: an electromagnetic transmitter with four transmission power supplies P 1 , P 2 , P 3 , P 4 , five sets of grounding electrodes A, B, C, D, F, and eight transmission wires LA, LB 1 , LB 2 , LC 1 , LC 2 , LD 1 , LD 2 , LE. The grounding electrode A and the grounding electrode B are connected to the transmission power supply P 1 via the transmission wire LA and the transmission wire LB 1 . The grounding electrode B and the grounding electrode C are connected to the transmission power supply P 2 via the transmission wire LB 2 and the transmission wire LC 1 . The grounding electrode C and the grounding electrode D are connected to the transmission power supply P 3 via the transmission wire LC 2 and the transmission wire LD 1 . The grounding electrode D and the grounding electrode E are connected to the transmission power supply P 4 via the transmission wire LD 2 and the transmission wire LE. The transmission wire LA and the transmission wire LB 1 are pulled from the electromagnetic transmitter to a position of the grounding electrode B in a twisted-pair manner, and the transmission wire LD 2 and the transmission wire LE are pulled from the electromagnetic transmitter to a position of the grounding electrode D in the twisted-pair manner. For twisted wires with opposite currents, the surrounding magnetic fields cancel each other out, and the inductance can be ignored.

Since the transmission power supplies P 1 , P 2 , P 3 , P 4 are isolated from each other, the issue of loop current leakage is therefore avoided. Assuming the inductance of the transmission wire is L and the grounding resistance is R, the current turn-off time is:

t off = - ln ⁡ ( 1 2 ) · L LOAD R LOAD ≈ 0.693 · L LOAD R LOAD . ( 1 )

It can be seen that the current turn-off time is only related to the grounding resistance and the wire inductance. Under the condition that the grounding resistance is determined, the inductance of 1 km long wire is divided into four small inductors with inductance of 0.5637 mH, and the turn-off time is reduced by about 78%.

As an implementation of the embodiment of the disclosure, as shown in FIG. 4 , the electromagnetic transmitter includes: a diesel generator, a three-phase rectifier, an inverter isolation power supply, a constant current voltage regulator, and an output inverter. The diesel generator is configured to provide 380 V three-phase alternating current. The three-phase rectifier is configured to convert the three-phase alternating current into 540 V direct current. The inverter isolation power supply is configured to acquire four sets of 1000 V isolated power supplies through an inverter bridge, four transformers, four high-frequency rectifier bridges, and an LC filter. The constant current voltage regulator is configured to control an output voltage to achieve constant current regulation. The output inverter is configured to generate the four output transmission power supplies P 1 , P 2 , P 3 , P 4 .

Further, the four transmission power supplies P 1 , P 2 , P 3 , P 4 are relatively independent, and according to the output load, duty cycle signals are dynamically adjusted by controlling switching devices Q 1 -Q 4 to maintain the output current constant.

The four transmission power supplies P 1 , P 2 , P 3 , P 4 are completely consistent in structure. Here, the transmission power supply P 4 is taken as an example to expand the connection relationship of the system, and other transmission power supplies are the same.

The diesel generator outputs three-phase electricity Va, Vb, and Vc (i.e., alternating current voltages Va, Vb, and Vc).

The three-phase rectifier utilizes the unidirectional conductivity of diodes to convert the three-phase alternating current voltages Va, Vb, and Vc into direct current. The three-phase rectifier consists of diodes D 1 -D 6 and capacitor C. Specifically, the anode of the diode D 1 is connected to the cathode of the diode D 2 and connected to the generator Va; the anode of the diode D 3 is connected to the cathode of the diode D 4 and connected to the generator Vb; the anode of the diode D 5 is connected to the cathode of the diode D 6 and connected to the generator Vc; the cathodes of the diodes D 1 , D 3 , and D 5 are connected and connected to one end of the non-polar capacitor C as the positive terminal of the three-phase rectifier power supply; the anodes of the diodes D 2 , D 4 , and D 6 are connected and connected to the other end of the non-polar capacitor C as the negative terminal of the three-phase rectifier power supply.

The inverter isolation power supply uses the switching method of an H-bridge, which is composed of controlled power switching devices to make the output alternate between positive and negative, converting direct current into alternating current. An isolated alternating current is obtained through an isolation transformer, then a rectification bridge is used to convert the alternating current back into direct current with larger ripple. Finally, an LC filter is employed to achieve low-ripple direct current. The inverter isolation power supply includes insulated gate bipolar transistors (IGBT) S 1 -S 4 , transformer T 4 , fast recovery diodes D 41 -D 44 , inductor L 41 , and capacitor C 41 . Specifically, the collector of the transistor S 1 is connected to the collector of the transistor S 3 and connected to the positive terminal of the three-phase rectifier power supply at one end of the non-polar capacitor C. The emitter of the transistor S 2 is connected to the emitter of the transistor S 4 and connected to the negative terminal of the three-phase rectifier power supply at the other end of the non-polar capacitor C. The emitter of the transistor S 1 is connected to the collector of the transistor S 2 and connected to the primary same-polarity terminal of the transformer T 4 . The emitter of the transistor S 3 is connected to the collector of the transistor S 4 and connected to the primary opposite-polarity terminal of the transformer T 4 . The anode of the diode D 41 is connected to the cathode of the diode D 42 and connected to the secondary same-polarity terminal of transformer T 4 . The anode of the diode D 43 is connected to the cathode of the diode D 44 and connected to the secondary opposite-polarity terminal of transformer T 4 . The cathodes of the diode D 41 and the diode D 43 are connected and connected to one end of the inductor L 41 , and the other end of the inductor L 41 is connected to one end of non-polar capacitor C 41 as the positive terminal of the inverter isolation power supply; the anodes of the diode D 42 and the diode D 44 are connected and connected to the other end of the non-polar capacitor C 41 as the negative terminal of the inverter isolation power supply.

The constant current voltage regulator is based on the principle of BUCK circuit, and the output voltage is proportional to the duty cycle of pulse-width modulation (PWM). The constant current voltage regulator is based on the PWM control method, and by adjusting the duty cycle, the purpose of constant current voltage regulation is achieved. The constant current voltage regulator includes IGBT Q 4 , diode D 45 , inductor L 42 , capacitor C 42 , and diode D 46 . The collector of the transistor Q 4 is connected to the positive terminal of the inverter isolation power supply at one end of non-polar capacitor C 41 , the emitter of the transistor Q 4 is connected to the cathode of the diode D 45 and connected to one end of the inductor L 42 , the other end of the inductor L 42 is connected to one end of non-polar capacitor C 42 and connected to the anode of the diode D 46 , and the cathode of the diode D 46 is the positive terminal of the constant current voltage regulation power supply. The other end of non-polar capacitor C 42 is connected to the anode of the diode D 45 and connected to the negative terminal of the inverter isolation power supply at the other end of capacitor C 41 as the negative terminal of the constant current voltage regulation power supply.

The output inverter uses the switching method of the H-bridge composed of controlled power switching devices to make the output alternate between positive and negative, converting direct current into alternating current. The output inverter includes power switching device K 4 , capacitor C 4 , power switching device K 41 , power switching device K 42 , power switching device K 43 , and power switching device K 44 . The emitter of the power switching device K 4 is connected to the collectors of the power switching devices K 41 and K 43 , and to the cathode of the diode D 46 of the constant current voltage regulation power supply. The collector of the power switching device K 4 is connected to one end of the capacitor C 4 , and the other end of the capacitor C 4 is connected to the emitters of the power switching devices K 42 and K 44 , and to the anodes of the diodes D 42 , D 44 , and D 45 of the constant current voltage regulation power supply. The emitter of the power switching device K 41 and the collector of the power switching device K 42 are connected as one terminal of output power supply P 4 to the transmission electrode. The emitter of the power switching device K 43 and the collector of the power switching device K 44 are connected as the other terminal of output power supply P 4 to another transmission electrode.

The typical transmission waveform for the SOTEM method is a 50% duty cycle bipolar square wave, observing the transient signal generated by the ground after the pulse is turned off. To achieve rapid switching of output current, the output inverter adopts bootstrap high-voltage clamping technique. The system connection relationship of power supply P 4 is illustrated in FIG. 5 as an example, and other power supplies follow similarly.

Due to the unidirectional conductivity of the diode D 46 , when the cathode voltage of the diode D 46 is higher than the anode, the diode D 46 is cut off; otherwise, it conducts. In the initial state, the voltage of bootstrap capacitor C 4 is equal to the constant current voltage regulation output voltage. Since the power supply P 4 is connected to the electrodes via long wires with significant inductance, after the inverter bridge turns off the transmission current, the reverse current through the wire inductance charges the capacitor C 4 . The charging process of the bootstrap capacitor C 4 is shown in FIG. 6 .

To achieve rapid switching of current, the bootstrap capacitor C 4 is first charged. When the charging voltage reaches a preset value, the control of power switching device K 4 is activated, using the voltage on the bootstrap capacitor C 4 to rapidly magnetize and demagnetize the inductive load of the output inverter. This enables the conversion of magnetic energy on the long wires and electrical energy on the capacitor, achieving high-bandwidth current change in the transmission. After the bootstrap capacitor C 4 is fully charged, the switching control signals for the switching devices K 41 -K 44 of the inverter bridge and the bootstrap switching device K 4 , and the transmission waveforms of the power supply P 4 and transmission current 14 , are shown in FIG. 7 .

Furthermore, a simulation model of the output inverter based on the bootstrap high-voltage clamping technique is constructed. The transmission voltage is set to 1000 V, with a 1 km long wire inductance of 2.5 mH, grounding resistance of 50 ohm (Ω), and a 1 kHz output frequency with a 50% duty cycle bipolar square wave. When the voltage of the bootstrap capacitor reaches 2000 V, the normal control of the power switching device K 4 is started. The control signal V 1 and V 2 from the output inverter bridge are “AND” operated, and then the result is “XOR” operated with its signal delayed by 30 microseconds (μs) to control the power switching device K 4 . The simulation results are shown FIG. 8 .

It can be seen that, at the beginning of startup, the voltage of the bootstrap capacitor gradually rises until stabilizing at 2000 V, after which normal operation begins. During the rise of the capacitor voltage, the current turn-off time is approximately 33 μs. Once the bootstrap capacitor voltage reaches 2000 V, the power switching device K 4 begins operating, at which point the current turn-off time is approximately 20 μs. The system has a significant effect of accelerating current switching.

The embodiments described above are only illustrative of the illustrated implementations of the disclosure and do not limit the scope of the disclosure. Under the inventive concept of the disclosure, any modifications or improvements made by those skilled in the art without creative effort shall fall within the protection scope defined by the claims of the disclosure.